void fitZ(){
  const double p[] = {39.2447, 34.9797, 33.9068, 32.8535, 31.7741, 30.7059, 29.6379, 28.57, 27.4997, 26.4316, 25.3732, 24.3079, 23.2403, 22.1799, 21.1137, 20.0491, 18.9849, 17.9255, 16.8658, 15.8129, 14.7671, 13.7219, 12.678, 11.6614};
  static const int n = sizeof(p)/sizeof(double);
  double z[n]{};
  z[0] = 36.;
  for(int i = 0; i < n - 1; i++) z[i + 1] = 32 - i;

  TGraph *g = new TGraph(n, p, z);
  g->SetMarkerColor(2);
  g->SetMarkerStyle(20);
  TCanvas *c = new TCanvas("c", "calibration of Z", 800, 600);
  g->Draw();
  g->Fit("pol1");
} // end function fitZ
